Intestinal toxicity produced by anti-folate treatment with amethopterin has seriously limited the use of this drug in chemotherapy of cancer and other diseases. The toxicity is thought to be related to inhibition of dihydrofolate reductase by amethopterin, which results in a block of key metabolic pathways and death of intestinal cells. The mechanism of the membrane transport of amethopterin (and other folate compounds), has been under study by the applicant using in vitro preparations from rat intestine and liver. These techniques and others described in the text will be applied to further analysis of the mechanisms of transport of amethopterin and other folate compounds in the intestine and liver. Characterization of these transport processes should provide the information needed to understand how transport may be controlled in order to reduce the interaction of the drug with its target enzyme, dihydrofolate reductase, in normal, proliferating intestinal cells.